Pressurized-water coolant nuclear reactor installations include the reactor vessel and a biological shield surrounding this vessel. The shield forms a cavity in which the vessel is supported and shields the latter's side wall and bottom. In addition to its biological function, the shield functions to contain the vessel in the event it ruptures, thus indicating that the biological shield should surround the vessel as closely as possible, leaving a space only for the normal thermal movement of the vessel.
However, if this minimum space between the shield and vessel is used, there is inadequate upward conduit space for the rapid release of the vessel's pressurized-water coolant in the event the vessel ruptures. In addition, such a small space does not leave enough room for the insertion of instruments between the shield and vessel for the purpose of inspecting the condition of the vessel's wall. It is to be understood that the pressure vessel is designed with a wide margin of safety. However, nuclear reactor installations must meet extremely severe demands concerning safety. The maximum hypothetical accident must be considered at all times.